The invention relates to a magnetic-field generating device that exhibits in particular permanent magnets in a Halbach arrangement. The magnetic field that has been generated exhibits a field-free line having a predetermined magnetic-field gradient in a predetermined plane that can serve as a scan plane for Magnetic Particle Imaging.
During Magnetic Particle Imaging (MPI), local concentrations of magnetizable nanoparticles are determined in an a priori unknown spatial distribution inside a patient. These nanoparticles are not radioactive and the MPI measurement method in addition does not require any ionizing radiation. For example super paramagnetic iron-oxide particles can be magnetized periodically by a drive field that can be changed periodically at a predetermined frequency, the magnetization of the particles being a non-linear function of the strength of the magnetic field. By detecting and analyzing the temporal behavior of the particle magnetization relative to the characteristic of the drive field, the particle concentration can be inferred.
For narrowing down the MPI measurement to small volumes of the patient, the drive field is superposed by a normally temporally constant selection field. The selection field has a zero at at least one predetermined point of the analysis region. Starting from this, a so-called field-free point, FFP for short, a selection field quickly increases in all directions so that the magnetizable nanoparticles reach magnetic saturation even at a short distance from the FFP. Nanoparticles at a great distance from the FFP then hardly react to the drive field and do not contribute significantly to the detected signal. The MPI measurement signal rather originates from the local surroundings of the FFP and informs on the local particle concentration that is present there.
As an alternative, it is also possible to generate instead of a single FFP a field-free line, FFL for short, for example using a suitable configuration of magnetic-field generating coils. Using an FFL instead of a single FFP has the potential to reduce the data acquisition duration by one order of magnitude. However, all magnetic particles of an object placed inside the coil arrangement along the FFL respond simultaneously to a change in the drive field so that evaluating the MPI measurement signals in terms of local concentrations that can then be assigned to individual voxels of the object, requires a numerically more complex effort than for only one FFP.
It is prior art to shift the position of an FFP or an FFL inside a region from which the MPI measurement data are to be obtained—the so-called field of view, FOV—by superposing a time-varying homogenous magnetic field on the static selection field. Specifically for the MPI having an FFL, this homogenous magnetic field is to be applied at right angles to the course of the FFL and on account of its field direction also determines along which plane the FFL can be shifted. As a result of, the scan plane of the MPI through which in the ideal case a patient is to be moved through at right angles, is established.
For the numerically more complex treatment, mentioned above, of the MPI measurement data when using an FFL it is important that the FFL can be rotated inside the scan plane about its center point, i.e., that the MPI signal detection can take place in a manner comparable to an x-ray CT measurement with a plurality of orientations of the FFL relative to the resting patient. The homogenous magnetic field that always serves the scanning lateral movement of the FFL in the scan plane, therefore up to now has to be generated by a coil pair that accompanies the rotary movement of the FFL also in terms of instruments, i.e. the coil arrangement has to be designed so as to be rotatable at least within limits. This represents a disadvantage both in terms of material wear and also handling of the high-voltage carrying leads to the coil pair.
The person skilled in the art knows so-called Halbach arrangements of permanent magnets. Such arrangements can in particular be designed as geometrical objects in which a plurality of permanent magnets, for example in rod shape, are present embedded in a predetermined arrangement. Such objects usually consist of a non-magnetic material matrix, are for example shaped bodies from a cured polymer such as e.g. polymethyl methacrylate (PMMA) and comprise strong permanent magnets based on rare-earth elements. The objects themselves then exhibit permanent magnetic fields whose flux densities in part areas of their surface can be larger than the largest flux density of any of the embedded permanent magnets. The objects comprising a Halbach arrangement at the same time also exhibit part areas of their surface that are virtually free from any magnetic field.
Among the most prominent examples for geometrical objects comprising Halbach arrangements are the Halbach cylinders. Such cylinders have an inside and an outside diameter and in this respect consist of a material that forms a thick cylinder jacket, i.e. a tube. A Halbach cylinder is characterized in that in the course of the jacket the magnetization direction of the jacket exhibits a constant rotation in the plane at right angles to the cylinder axis. Since the jacket is closed, the magnetization direction returns to its starting value in the case of one rotation of the entire cylinder jacket. For clarification, reference is made to FIG. 1. (Source: “Halbach cylinder” by Own work by en:User:Hiltonj—originally uploaded to the English language Wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons—https://commons.wikimedia.org/wiki/File:Halbach_cylinder.png #/media/File:Halbach_cylinder.pnq).
When expressed in plane polar coordinates R, φ, the—two-dimensional—magnetization vector of a Halbach cylinder jacket is described by {right arrow over (M)}=M0 (cos kφ, sin kφ), k having to be a natural number and often being described as a Halbach order.
FIG. 1 shows diagrams of the Halbach cylinders of the first four orders with the predetermined magnetization directions of the jacket material and the magnetic-field directions, resulting therefrom, in the—material-free—inside spaces of the cylinders. Obviously, a Halbach cylinder of the Halbach order k=2 is of particular practical interest since it is capable of providing a relatively strong homogenous magnetic field at right angles to the cylinder axis in its interior.
In principle, the magnetic field of an ideal Halbach cylinder can be precisely calculated analytically. However, since in practice Halbach cylinders are produced from a finite number of permanent-magnetic segments anyway and consequently the rotation of the magnetization direction in the cylinder jacket takes place in discrete steps, the analytical calculation can hardly be handled for real devices. Rather the numeric simulation of the magnetic field inside the cylinder is resorted to. Here it is quickly recognized that the homogeneity of the magnetic field for a cylinder of the Halbach order k=2 for a real arrangement is of course only an approximation and in this case is not even a good one for the vicinity of the cylinder top surfaces. As a matter of principle, the cylinder height may not be selected to be too small if the postulated geometry of the magnetic field is to be achieved at least in part areas of the cylinder inside.
In comparison to the inside diameter of the cylinder, the cylinder height is usually large, often even larger than the outside diameter. Directing and focusing charged particle beams is among the known applications of Halbach cylinders of the Halbach order k=3.
It is in particular Knopp et al. “Generation of a static magnetic field-free line using two Maxwell coil pairs”, APPLIED PHYSICS LETTERS 97, 092505, 2010, that mentions the current-less generation of an FFL in the quadrupole field of a Halbach cylinder of the Halbach order k=3 as a possibility for the MPI. However, this FFL at first is on the axis of symmetry of the Halbach cylinder along which e.g. a patient would have to be pushed in. In this respect, the FFL has at first to be rotated by 90° into a suitable scan plane, and then also two coil pairs for shifting the FFL in the scan plane have to be arranged. The work by Knopp et al. circumvents this by replacing the Halbach cylinder by two pairs of Maxwell coils, it now being possible to move the patient along an axis through one of the coil pairs.
Halbach cylinders having the Halbach order k=2 are already applied as current-free generators of homogenous magnetic fields even in the case of magnetic resonance tomography—MRT—, as can be gathered from the printed matter US 2015/177343 A1. It shall only be emphasized here that the field of view is in the cylinder inside in the work mentioned.
There also exist further applications of Halbach cylinders in magnetic gears. Here a plurality of Halbach cylinders—usually of differing Halbach orders—is arranged placed inside each other concentrically. A rotational movement of one of the cylinders is transmitted contact-free to the other as a result of the magnetic interaction. In magnetic gears, the cylinder height is in part rather small, often markedly smaller than the outside diameter.
The printed matter EP 1876462 A1 uses a plurality of stacks of Halbach rings that are placed inside each other concentrically and can be rotated relative to each other so that the magnetic field can be fine-tuned as precisely as possible inside the concentric stacks.